Demand for low power electronic devices continues to grow. Circuit designers are increasingly lowering the power provided to electronic devices. However, lower power may have an adverse effect on the dynamic range of components of an electronic device. For example, if an amplifier or comparator device is powered by a lower supply voltage (e.g., 1.8 volts), or lower current supply it limits the range of input signals that can be applied to the device. In order to compensate for the lower power supply in an integrator circuit, for example, a feedback capacitor may need to be larger to accept higher input currents with such a low supply voltage. However, the larger feedback capacitor makes the integrator gain lower and, when the input current signal is lower, the output signal may not be large enough to be detected by a following stage.
Output noise may also be generated, for example, due to the thermal characteristics of the electrical components (e.g., transistors) of the amplifiers used in the electronic devices, such as integrators. The noise may be propagated upstream thereby causing unacceptable output noises.
Circuits that perform integration functions are known in the art as integrators. In a conventional integrator as shown in FIG. 1, the input current IIN is integrated across the capacitor CFB. In other words, as IIN changes over time, the voltage VOUT changes inversely to the input current signal.
In more detail, when reset switch SWRESET is CLOSED, the feedback capacitor CFB is discharged, the voltage VIN at the integrator input and output voltage VOUT are reset to equal VREF by the response of the amplifier A1 (after reset VIN=VREF=VOUT).
When integrating, the reset switch SWRESET is OPEN, and the input signal IIN is applied to the integrator input briefly causing voltage VIN to fluctuate from voltage VREF. The amplifier A1 responds to this fluctuation by outputting a signal to VOUT, so VIN will return to the value VREF. At any time T during integration, the output voltage VOUT may be approximately equal to VREF−(IIN*T/CFB), where T is the time while integrating the current signal IIN.
Generally, the amplifier A1 outputs an amplified voltage VOUT proportional to the difference between VREF and VIN. However, the amplified voltage output by the amplifier A1 is limited by power supply voltage VDD to the amplifier A1. Amplifier A1 cannot output a voltage higher than VDD or lower than ground as shown in FIG. 1. In other words, VOUT will not be greater than VDD.
As mentioned above, circuit designers aim to design circuits having low power and low noise, e.g., thermal noise. The circuit designs require a tradeoff between low power and higher noise, because larger supply current is needed for reducing thermal noise associated with transistors within the amplifiers. Additionally, an external sensor, which may be the input current source IIN, may require higher voltage potentials for proper bias conditions. In the conventional integrator, such as those used in imaging applications, the input current is integrated over time and a representative output voltage is provided. Noise introduced by the amplifiers into the output voltage will be propagated to further devices. Therefore, it is desirable to reduce the amount of noise introduced by the amplifiers of the integrator.
Noise from amplifiers may result from higher temperatures. The higher temperature (for example, approximately 85 degrees C.) can increase thermal noise. One method of reducing thermal noise is to raise the supply current provided by the voltage source of VDD. The lower power consumption of the amplifier by using a lower supply voltage also results in lower noise due to a reduced temperature of the amplifier.
One known attempt to address this problem has been to put amplifiers in series as shown in FIG. 2. The integrator of FIG. 2 includes a low noise amplifier (LNA) A1, a second amplifier A2, and a feedback capacitor CFB. The LNA A1 that is coupled to a reference voltage VREF on a first input and a current source IIN on a second input. The voltage at the second input is labeled VIN. The LNA A1 is powered by a voltage source VDDL. The second amplifier A2 (not necessarily a low noise amplifier) has inputs coupled to the outputs of LNA A1, and is powered by a second voltage source VDDH. The feedback capacitor CFB is connected to an output of the second amplifier A2 and the VIN node.
While amplifier A2 may be a transconductance amplifier. However, the noise contribution of amplifier A2 is divided by the gain of amplifier A1. Therefore, the noise generated by amplifier A2 is not as problematic. Noise generated by amplifier A1 may be propagated through to VOUT. The gain of amplifier A1 may be between 5 and 20. The power supply voltage VDDL may be less than 5 volts.
In contrast to amplifier A1, amplifier A2 may be allowed to be a higher noise source by having a lower supply current and a higher supply voltage VDDH, which may be equal to or greater than 5 volts. The configuration shown in FIG. 2 realizes lower power, and lower noise with a wider dynamic range than the conventional integrator of FIG. 1. However, the input common mode range, represented by VIN, is limited to a lower input potential because the amplifier A1 is supplied with a lower supply voltage VDDL.
Since the supply voltage VDDL of amplifier A1 is low, the reference voltage VREF must be either equal to or less than VDDL. In the integrator shown in FIG. 2, the input common mode range VIN is dependent upon the value of VREF, which is limited by Supply voltage VDDL. Due to this limitation, the above configuration may not be suitable for use when the input voltage VIN and the reference voltage VREF need to be higher. For example, when input current source IIN is an external sensor that requires higher potential for its proper bias condition, the integrator confirmation of FIG. 2 that supplies the input current signal may not be appropriate.
The input device IIN may be a customer device, such as a photodiode. A photodiode typically supplies between 0-5 volts. If 5 volts is applied to amplifier A1, VDDL would have to supply at least that amount of voltage, which would result in higher power consumption of the circuit. In addition, the noise associated with amplifier A1 may be dominated by thermal noise. The thermal noise of amplifier A1 may be reduced if more supply current is consumed. Therefore, in order for amplifier A1 to achieve both low power consumption and low noise, less voltage and more supply current, respectively, is needed to be supplied from VDDL.
Accordingly, another more flexible solution is needed. There is a need for a low power, low noise integrating device that provides acceptable bias conditions.